The invention relates to a method of performing electrical impedance tomography measurements and electrode arrangements for use therein.
The invention is applicable generally to the imaging of electrical impedance variations in conductive media, and especially in volume conductors such as parts of the human body, the ground, pipelines, and so on.
Electrical impedance tomography (EIT), also known as xe2x80x9capplied potential tomographyxe2x80x9d (APT), is used to provide images of spatial variations in electrical impedance within a conductive volume conductor. The electrical impedance changes are measured by providing an array of electrodes about the volume to be imaged. A stimulus current is supplied to each pair of electrodes in turn and the resulting potential differences recorded between pairs of the remaining electrodes. The process is repeated until all of the independent combinations of stimulation/recording are exhausted. The measurements are used to determine the transfer impedance changes and construct an image of the volume. In contrast to X-ray computer tomography applications, where the paths of photons through a body are straight lines, the current paths in EIT are functions of an unknown electrical conductivity distribution. This gives rise to a non-linear image reconstruction problem. Nevertheless, algorithms have been devised for converting the series of measurements into an image of the electrical impedance distribution within the body at a sufficiently high rate that changes associated with respiratory and cardiac functions, for example, can be monitored.
The quality of the image depends upon the sensitivity of the measuring system, which typically varies with distance from the electrodes. In clinical applications, for example, where the electrodes are distributed around the body part to be measured, such as the thorax, difficulties may be encountered in resolving impedance changes which occur deep in the body. Most attempts to improve image quality have focused upon the reconstruction algorithms, as disclosed, for example, in U.S. Pat. Nos. 4,617,939, 5,381,333 and 5,465,730. Limited attention has been devoted to the electrode arrangement. For example, a study of various electrode configurations was described by Booth et al. in xe2x80x9cA Comparison of Three Electrode Configurations for Electrical Impedance Tomographyxe2x80x9d, IEE Colloquium on xe2x80x9cInnovation in Instrumentation for Electrical Tomographyxe2x80x9d, pp. 11/1, 11/3, 1995. British patent application number 2,257,530 disclosed electrodes in a circular array or xe2x80x9crosettexe2x80x9d placed upon a substantially flat part of the body, such as the chest. This was said to be especially desirable when imaging the heart, since impedance changes occurring in the heart during the cardiac cycle will be larger than those occurring in the region of the skin.
Little attention has been paid to the way in which the measurements themselves are made. There are two kinds of procedure for taking the measurements. One involves applying stimulation currents simultaneously to a ring of electrodes to generate a current pattern and simultaneously measuring the corresponding voltage distribution around the same or adjacent electrodes. An example of such an approach is described in an article by P. Hua et al. entitled xe2x80x9cAn Electrical Impedance Tomograph using Compound Electrodesxe2x80x9d, presented at the IEEE Engineering in Medicine and Biology Society 11th. Annual International Conference, 1989. Hua et al. described experiments with thirty-two compound electrodes encircling a body to be measured. Each compound electrode comprised an inner electrode surrounded by an outer electrode with an annular space between them. In some cases, Hua et al. short-circuited the inner and outer electrodes and used them for both stimulation and recording. In other cases, Hua et al. injected a spatially-sinusoidal current pattern into the ring of 32 outer electrodes and recorded the corresponding generally sinusoidal voltage distribution around the ring of 32 inner electrodes. This approach requires 32 adjustable current generators which must be adjusted individually to give the required pattern as accurately as possible, making it difficult to obtain repeatable measurements accurately. Also, the equipment is complex and costly.
An example of the other kind of procedure is described in U.S. Pat. No. 4,617,939 (Brown et al.) issued October 1986. Brown et al. describe positioning 16 electrodes around a body, applying a current to a first pair of the electrodes and recording the potential difference between every other pair of the remaining electrodes. As also noted by Hua et al. (supra), significant contact impedance between the electrodes and the body surface militates against measurement sensitivity, so Brown et al. avoid using the same electrode for both stimulation and recording. Brown et al. repeat the procedure, applying current to pair of electrodes in turn. For each pair, Brown et al. measure voltages at all of the remaining electrodes. In practice, such a procedure would not be entirely satisfactory because many of the voltage measurements would be comparable with noise levels.
The present invention seeks to mitigate the disadvantages of these known EIT systems and provide an improved electrical impedance tomography procedure with enhanced sensitivity to changes in the electrical conductivity distribution inside the body of interest.
According to one aspect of the invention, a method of determining electrical impedance tomography of a conductive volume comprises the steps of selecting, in turn, each of a plurality of pairs of locations upon a surface of the conductive volume, supplying a stimulation current to the surface by way of the selected pair of locations, and recording, for each selected pair of locations, a resulting potential difference between at least one pair of the remaining locations, characterized in that, the steps of, for a particular pair of locations stimulated, selecting from among the remaining locations, for recordal, the pair or pairs of locations which, if stimulated, would produce an electric field with vectors most closely aligned with the corresponding vectors of the electric field produced by stimulation of said particular pair of locations, and recording said potential difference at the or each said pair of locations so selected for recordal.
The above method may be performed using a plurality of electrode means, each disposed upon or adjacent said surface at a respective one of said plurality of locations, each electrode mean being used to apply stimulation current to, or record potential at, said respective one of said locations.
Each electrode means may comprise two electrodes, that are closely located spatially, one for stimulation and the other for recording. One electrode may be disposed inside the other in a plane which, in use, will be parallel to the said surface. The stimulation current could then be applied to two outer electrodes and the potential difference measured at a said selected pair comprising the corresponding two inner electrodes. Conversely, the stimulation current could be applied to the two inner electrodes and the potential difference remeasured at the corresponding outer electrodes.
Alternatively, each electrode means may comprise a first electrode and a second electrode spaced apart from each other in a direction which, in use, will be substantially normal to the said surface. The stimulation current could be applied by way of two of said second electrodes and the potential difference measured between two of said first electrodes, or vice versa.
According to another embodiment of the invention, a method of measuring electrical conductivity distribution within a conductive volume includes the steps of positioning adjacent a surface of the body an array of electrodes, with a first group of the electrodes closer to the surface than a second group of the electrodes, applying a stimulation current to selected pairs of electrodes in turn and measuring, for each pair stimulated, corresponding electrical potential differences produced between pairs of the remaining electrodes, and processing the measured potential differences to determine electrical conductivity variations within the body.
According to another aspect of the present invention, an electrode arrangement for an electrical impedance tomography system comprises a plurality of electrodes and support means for supporting the electrodes adjacent a surface of a conductive body the conductivity of which is to be mapped by the system, the arrangement being such that a first group of said plurality of electrodes will be closer to the surface than a second group of the plurality of electrodes.
According to yet another aspect of the invention, an electrode arrangement for an electrical impedance tomography system comprises a plurality of electrodes in an array mounted in a support medium for supporting the electrodes adjacent a surface of a volume to be imaged, the support medium having anisotropic conductivity, its conductivity in a direction that, in use, is normal to the surface, being significantly greater than its conductivity in a transverse direction.
According to a further embodiment of the invention, an electrode arrangement for an electrical impedance tomography system comprises an array of electrodes fixed spatially relative to each other in a rigid support medium, and an interface medium for interfacing the rigid support to a surface of a conductive volume to be measured, the interface medium being conductive so as to connect the electrodes electrically to the surface and pliable so as to conform to variations in relief of the surface.
An advantage of this electrode arrangement of such further aspect, as compared with electrodes which are simply attached directly to the volume, is that the relative position of the electrodes, and the dimensions of the array, are fixed, which facilitates static image reconstruction of the image.
According to yet a further aspect of the invention, an electrode arrangement for use in electrical impedance tomography comprises a plurality of electrode means each formed by a stimulation electrode and a recording electrode closely located spatially. In one embodiment of this aspect, the electrode means comprises an inner electrode surrounded by an outer electrode with an annular channel between them. Electrically insulating sealing means may be provided in the annular channel for contacting said surface, in use, and insulating the inner electrode electrically from the outer electrode.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, taken in conjunction with the accompanying drawings.